Type II oestrogen binding site is associated with the major 4S oestrogen receptor isoform in breast tumours

Bisphenol a binds to the low-affinity estrogen binding site

Environmental estrogens are suspected of being involved in the current increase in the incidence of human reproductive malfunctions, such as a decrease in male reproductive capacity and an increased incidence of breast cancer in women. The influences of these compounds have been proposed to be mediated through binding to macromolecules, such as estrogen receptor alpha or beta. In this study we examined whether the low-affinity Type II estrogen binding site (Type II EBS), originally identified in the rat uterus, is a possible mediator of environmental estrogens such as bisphenol A (BPA). Analysis of BPA's binding to an enriched fraction of Type II EBS, using a competition assay, indicated that BPA was able to compete with estradiol in binding to this site. At a concentration of 10-15 microM (comparable to that required to induce uterine proliferation), BPA inhibited the binding of estradiol to Type II EBS by greater than 50%. The binding affinity of BPA for the Type II EBS was only 8-10-fold lower than that of the synthetic estrogen diethylstilbestrol. The binding of BPA to Type II EBS appeared specific to BPA, in that endosulfan, another environmental estrogen, failed to displace estradiol from the site. A comparison of the relative binding affinities of BPA for rat uterine estrogen receptor alpha to that of the Type II EBS implies that BPA preferentially binds to the Type II EBS.

Glucocorticoid receptors in the euryhaline teleost Anguilla anguilla

To determine the importance of glucocorticoids in the salt water adaptation of European yellow eel we have evaluated the concentration, affinity and physical properties of glucocorticoid receptors (GR) in gill from both sea water- (SW) and freshwater-adapted (FW) animals. Using ligand binding techniques we demonstrated that high affinity GR were present in both cytosolic and nuclear fractions obtained from whole gill. Isoelectric focusing (IEF) of branchial GR indicated the presence of two distinct species, with pI values of 6.1 and 6.7. The form at pI 6.7 sedimented with a Svedberg constant of 4S on glycerol density gradients while the pI 6. 1 sedimented in fractions corresponding to 9S. Treatment of the pI 6. 1 form with urea (4 M) resulted in generation of the form with pI 6. 7. The evidence thus suggested that the oligomeric urea-sensitive form (pI 6.1) contained a form of GR which, as a monomer, focused at pI 6.7. IEF revealed the same concentrations of the pI 6.7 form in both SW and FW. However, there was significantly more (3-fold) pI 6. 1 isoform in FW than in SW, and this form decreased gradually during the course of seawater transfer. A transient increase of the nuclear-bound GR was also observed during SW adaptation. The balance between these forms could represent a dynamic parameter with important implications regarding GR function and gill responses to cortisol in salt water adaptation in teleosts.

Biphasic Scatchard plots of oestrogen receptors are associated with low pS2 levels in human breast cancers

In a series of 100 breast tumours, oestrogen receptors (ER) were analysed by Scatchard plots, progesterone receptors (PR) by enzyme immunoassay and pS2 by an immunoradiometric assay. Scatchard analysis gave information on receptor heterogeneity in that there was a large variation in Kd values obtained, from 0.001-2.95 nM. This variation was largely confined to tumours containing less than 70 fmol receptor per mg protein, while tumours with higher receptor concentrations were a more homogeneous population with low Kd values. An obvious correlation between ER and PR was found; moreover, pS2 was correlated to both ER and PR. In addition, 20 of the 100 tumours gave biphasic Scatchard plots, indicating the presence of at least two oestrogen-binding moieties, with Kd values and concentrations both in the range of those of receptors. The tumours displaying biphasic Scatchard plots had very low pS2 expression, regardless of ER concentrations; this was not true for PR. It is suggested that variability in responses to endocrine therapy may be related to the heterogeneity of the ER present in breast tumours.

Growth inhibitory effects of flavonoids in human thyroid cancer cell lines

Previous studies have indicated that flavonoids exhibit antiproliferative properties on some hormone-dependent cancer cell lines, such as breast and prostate cancer. In the present study, the effects of some selected flavonoids, genistein, apigenin, luteolin, chrysin, kaempferol, and biochanin A on human thyroid carcinoma cell lines, UCLA NPA-87-1 (NPA) (papillary carcinoma), UCLA RO-82W-1 (WRO) (follicular carcinoma), and UCLA RO-81A-1 (ARO) (anaplastic carcinoma) have been examined. Among the flavonoids tested, apigenin and luteolin are the most potent inhibitors of these cell lines with IC50 (concentration at which cell proliferation was inhibited by 50%) values ranging from 21.7 microM to 32.1 microM. The cells were viable at these concentrations. Using NPA cells known to be estrogen receptor positive (ER+), it was shown that no significant [3H]-E2 displacement occurred with these flavonoids at the IC50 concentration. In WRO cells that are known to have an antiestrogen binding site (AEBS), biochanin A caused a stronger inhibitory growth effect (IC50 = 64.1 microM) than in NPA and ARO cells. In addition, it was observed that biochanin A has an appreciable binding affinity for the AEBS as indicated by the displacement of [3H]-tamoxifen from the WRO cells. In summary, flavonoids have potent antiproliferative activity in vitro against various human thyroid cancer cell lines. The inhibitory activity of certain flavonoid compounds may be mediated via the AEBS and/or type II EBS. The observation that ARO cells that lack both the AEBS and the ER are effectively inhibited by apigenin and luteolin suggest that other mechanisms of action are operative as well. The present study suggests that flavonoids may represent a new class of therapeutic agents in the management of thyroid cancer.

Human larynx expresses isoforms of the oestrogen receptor

Commercially available enzyme immunoassays (EIAs) were used for oestrogen (ER) and progesterone (PR) receptor determination in the cytosol fraction of 118 human larynx cancer specimens and in the corresponding histologically proven non-malignant tissues. Fifty-one ER positive cancerous samples had corresponding non-cancerous tissues also expressing the receptor. A high resolution isoelectric focusing (IEF) technique followed by immunoblotting with the H222 anti-ER monoclonal antibody was used to evaluate the presence of ER isoforms in the 51 ER positive human larynx cancer specimens and in their corresponding non-malignant tissues. In both tissues, four ER isoforms were detected, with isoelectric points (pI) similar to those obtained in breast and endometrium carcinomas (6.1, 6.3, 6.6 and 6.8). A significant difference in the expression of ER isoforms between cancerous and non-cancerous tissue was found; precisely, the 94.1% of the ER positive non-malignant specimens co-expressed the four isoforms while they were detected in only the 35.5% of the malignant specimens (P < 0.0001 by Fisher's exact test). In larynx cancer, the concentration values of ER and PR did not correlate, nevertheless tumours co-expressing the four ER isoforms had PR levels significantly higher than those which did not (P = 0.02 by Mann-Whitney Wilcoxon sum rank test). To investigate the possibility that the isoforms of the monomeric 4S form of the ER (those with pI 6.3, 6.6, and 6.8) could dimerise, a cold agarose gel electrophoresis technique was used on IEF-separated ER isoforms. In summary, the evidence shows that all the isoforms are able to form homodimers and that the isoforms at pI 6.3 and 6.8 are able to dimerise with that at pI 6.6 but, under the same experimental conditions, they do not form the 6.3/6.8 heterodimer. It was concluded that: (1) the four isoforms of the ER are co-expressed by the non-malignant human larynx and the cancer loses the capacity to express some of them; (2) the complete complement of ER isoforms (all four) is needed for PR expression; (3) the monomeric 4S isoform with pI 6.6 has the capacity to form homo- and heterodimers, while the remaining two are only able to homodimerise.

A low-affinity estrogen-binding site in pregnant rat uteri: analysis and partial purification

We have identified a low-affinity (type II) estrogen-binding site (EBS) that is expressed at high levels during pregnancy in rat uteri. Although this activity was detectable in nonpregnant rat uteri, it was present in amounts (0.094 pmol/g of uteri) that were severalfold lower than the high-affinity type I estrogen receptor (0.57 pmol/g of uteri). During pregnancy, at 19-20 days of gestation, the low-affinity type II EBS became the major (> or = 88%) estrogen-binding site in rat uteri. The increase in the level of low-affinity EBS (7.9 pmol/g) in uteri was approximately 85-fold with an approximately 20-fold increase in the specific activity (0.39 pmol/mg) of this form, whereas the high-affinity form remained relatively unchanged. We report here a method of purification of type II EBS from pregnant rat uteri and present an analysis of its DNA and steroid-binding properties. Estradiol-binding studies and Scatchard analysis showed that the type II EBS had an apparent estradiol-binding affinity of > or = 24 nM. Gel filtration and SDS/PAGE analysis indicated that the type II EBS was a monomeric 73-kDa protein. The estradiol binding remained apparently uninhibited in the presence of a large excess of tamoxifen, nafoxidine, or dihydrotestosterone. Estradiol, diethylstilbestrol, and quercitin (a type II EBS-specific inhibitor) competed efficiently. The purified low-affinity EBS did not have sequence-specific DNA-binding activity with the estrogen-responsive element, which indicated that it differs in function from the type I estrogen receptor.